Cellulosic biomass is a significant renewable resource for the generation of sugars. Fermentation of these sugars can yield numerous end-products such as fuels and chemicals. While the fermentation of sugars to fuels such as ethanol is relatively straightforward, the hydrolytic conversion of cellulosic biomass to fermentable sugars such as glucose, generally referred to as “saccharification,” is difficult because of the crystalline structure of cellulose and its close association with lignin (Ladisch et al., 1983, Enzyme Microb. Technol. 5:82). Pretreatment, by methods including, but not limited to, mechanical and chemical treatments increases the susceptibility of cellulose to hydrolysis, presumably by breaking the lignin seal and disrupting the crystalline cellulose structure. This step may be followed by the enzymatic conversion of cellulose to glucose, cellobiose, cello-oligosaccharides and the like using enzymes that break β-1-4 glycosidic bonds of cellulose. These enzymes are collectively referred to as “cellulases.”
Cellulases are divided into three sub-categories of enzymes: 1,4-β-D-glucan glucanohydrolase (“endoglucanase” or “EG”); 1,4-β-D-glucan cellobiohydrolase (“exoglucanase”, “cellobiohydrolase”, or “CBH”); and β-glucosidase (“β-D-glucoside-glucohydrolase”, “cellobiase” or “BG”). See Methods in Enzymology, 1988, Vol. 160, p. 200-391 (Eds. Wood, W. A and Kellogg, S. T.). These enzymes act in concert to catalyze the hydrolysis of cellulose containing substrates. Endoglucanases randomly attack the interior parts and mainly the amorphous regions of cellulose, mostly yielding shorter cellulose chains. Exoglucanases incrementally shorten the glucan molecules by binding to the glucan ends and releasing mainly cellobiose, a water-soluble β-1,4-linked dimer of glucose, from the ends of the cellulose polymer. β-glucosidases split the cellobiose into two units of glucose.
Most cellulases have a multidomain structure consisting of a core domain separated from a cellulose binding domain (CBD) by a linker peptide (Suumakki et al., 2000, Cellulose 7:189-209). The core or catalytic domain contains the active site (van Tilbeurgh et al., 1986, FEBS Lett. 16:215; Tomme et al., 1988, Eur. J. Biochem. 170:575-81).
There are several types of microorganisms that produce cellulases. These include fungi, actinomycetes, and bacteria. Cellulases from strains of the filamentous fungi Trichoderma sp., Penicillium sp., Myceliophthora sp. and Chrysosporium sp. have been particularly productive in hydrolyzing cellulose, and cellulases derived from these strains have been previously used to hydrolyze cellulose. However, the cost of producing these enzymes along with their hydrolytic inefficiency under certain industrial conditions has been a drawback.
In order to maximize the hydrolysis of cellulosic substrates and enable commercial routes to end-product production (e.g., biofuels), it would be highly desirable to develop new cellulases and particularly new endoglucanases useful in the saccharification of biomass. The invention described herein fulfills these and other needs, as will be apparent upon review of the following disclosure.
In addition to being useful in the hydrolysis of biomass feedstock, cellulases have other industrial applications. Cellulases are useful in the pulp and paper industry, the textile industry, as detergent components, and as additives in animal feeds. The cellulases of the present invention may be useful in these applications as well.